In a multicarrier transmission system of multiplexing data symbols with respect to a plurality of subcarriers as in the OFDMA (Orthogonal Frequency Division Multiple Access) to simultaneously transmit a plurality of carriers corresponding to respective subcarriers, the plurality of carriers are caused to be of the same phase so that a high power peak takes place. As a result, PAR (Peak to Average power Ratio) becomes large.
A multicarrier signal with a high PAR is greatly distorted by an amplifier with the non-linear input/output relationship in transmitting such a multicarrier signal so that receiving characteristic in receiving such a multicarrier signal is disadvantageously deteriorated. In order to linearly amplify a multicarrier signal with a high PAR, it is necessary to set a large input back-off at a transmitter. However, when an input back-off is enlarged, the maximum transmission power is limited and the limitation of the transmission power directly affects broadness of coverage. In order to attain broad coverage in the multicarrier transmission system, PAR in a multicarrier signal is required to be reduced in order to have ability to attain linear amplification by a small back-off amount.
As a method of reducing PAR, there is a method of multiplying respective subcarrier components by phase rotation coefficients such that carriers are not caused to be mutually of the same phase. For example, in R. W. Bauml et al. “Reducing the peak-to-average power ratio of multicarrier modulation by selected mapping,” Electrics Letters, Vol. 32, No. 22, pp. 2056-2057, October 1996, there is disclosed a method in which a plurality of phase rotation sequences generated at random are prepared to multiply the same data symbol by phase rotation coefficients forming the plurality of phase rotation sequences to thereby generate a plurality of transmission signals with different transmission waveforms to select a signal of which PAR becomes minimum from these transmission signals thus to reduce PAR. A method of reducing PAR by Bauml et al. will be described below with reference to FIGS. 1 and 2.
Signal generating apparatus 2000 in the related art shown in FIG. 1 includes data symbol information generator 2001, phase rotation sequence generator 2002, data sequence generator 2003, frequency assignment information generator 2004, transmission signal generator 2005, and minimum peak power signal selector 2006. Here, transmission signal generator 2005 includes, as shown in FIG. 2, duplicator 2101, N pieces of rotation sequence multipliers 2102, N pieces of channel allocators 2104, and N pieces of IFFT (Inverse Fast Fourier Transform) units 2106.
In signal generating apparatus 2000 shown in FIG. 1, data symbol information generator 2001 generates data symbol information SDI corresponding to the number of data symbols multiplexed with respect to E subcarriers among F subcarriers which are equal to the number of points of inverse Fourier transform to output data symbol information SDI thus generated, wherein F is an integer equal to 2 or more, and E is a natural number equal to F or less. Phase rotation sequence generator 2002 generates the first to the N-th phase rotation sequences SRT-1 to SRT-N each including E phase rotation coefficients corresponding to data symbol information SDI on the basis of data symbol information SDI to output those phase rotation sequences. Data sequence generator 2003 generates data symbol sequence SDT including E data symbols corresponding to data symbol information SDI on the basis of data symbol information SDI to output data symbol sequence SDT thus generated. Frequency assignment information generator 2004 generates, as frequency assignment information SCA, information corresponding to frequency channels of the first to the M-th subchannels each including D successive subcarriers on the basis of data symbol information SDI to output these information, wherein D and M are natural numbers to satisfy the relation expressed as DXM=E.
In transmission signal generator 2005, as shown in FIG. 2, duplicator 2101 serves to duplicate data symbol sequence SDT by N-sequences to output the first to the N-th data symbol sequence SDT-1 to SDT-N. Rotation sequence multiplier 2102 serves to sequentially multiply data symbols respectively forming the i-th data symbol sequence SDT-i by phase rotation coefficients forming the i-th phase rotation sequence to thereby generate the i-th phase multiplication data sequence SCR-i including E data symbols by which phase rotation coefficients are multiplied to output the i-th phase multiplication data sequence SCR-i, wherein i is a natural number equal to N or less. Channel allocators 2104 to 2105 serve to multiplex data symbols of the i-th phase multiplication data sequence SCR-i with respect to a subcarrier corresponding to frequency assignment information SCA to allow the remaining (F-E) pieces of subcarrier components to be zero to generate the i-th subcarrier multiplexed signal Scc-i including F subcarriers in total to output the i-th subcarrier multiplexed signals Scc-i. IFFT unit 2106 serves to perform inverse Fourier transform of F-points with respect to the i-th subcarrier multiplexed signal Scc-i to generate transmission candidate signal sequence Svs-i on the time axis including F samples to output transmit candidate signal sequence Svs-i thus generated.
Returning to FIG. 1, minimum peak power signal selector 2006 of signal generating apparatus 2000 selects a signal of which peak power becomes minimum from the first to the N-th transmission candidate signal sequences SVS-1 to SVS-N to output the signal thus selected as a transmission signal (multicarrier signal SPM). This transmission signal is transmitted from, e.g., antenna (not shown).
The transmitting apparatus constructed in this way serves to select a signal of which peak power becomes minimum from a plurality of signals generated by respectively performing inverse-Fourier transform on signals obtained by multiplying subcarrier components by a plurality of phase coefficients, thereby making it possible to reduce peak power in a multicarrier signal.